Should the mechanical index be revised for ARFI imaging?

The mechanical index (MI) quantifies the likelihood that exposure to diagnostic ultrasound will produce an adverse biological effect by a nonthermal mechanism. The current formulation of the MI is based on inertial cavitation thresholds in two liquids, water and blood, as calculated by a formalism assuming very short pulse durations. Although tissue contains a high proportion of water, it is not a liquid but a viscoelastic solid. Further, acoustic radiation force impulse imaging employs high-intensity pulses up to several hundred acoustic periods long. The effect of these differences was studied in water, blood and five representative tissues.